Drift Cancellation Technique for Use in Clock-Forwarding Architectures

ABSTRACT

A circuit includes a frequency synthesizer, N phase mixers coupled to the frequency synthesizer, a plurality of receivers, and a calibration circuit. The frequency synthesizer is to receive a reference clock signal and is to output a primary clock signal. A respective phase mixer in the N phase mixers is to output a respective secondary clock signal having a corresponding phase. A respective receiver in the plurality of receivers is coupled to two of the N phase mixers, and at a respective time is to receive data in accordance with the respective secondary clock signal from one of the two phase mixers coupled to the respective receiver. The calibration circuit is to calibrate a secondary clock signal output by a respective phase mixer in the N phase mixers by adjusting the phase of the secondary clock signal of the respective phase mixer.

FIELD

The subject matter disclosed herein relates generally to circuits foruse in integrated circuits, and in particular, to circuits andassociated methods for canceling drift in devices that include aclock-forwarding architecture.

BACKGROUND

Many communication systems include devices that transmit and/or receivesignals synchronously, i.e., in accordance with one or more clocksignals. In some systems, the one or more clock signals may be generatedbased on data patterns that correspond to the signals. In other systems,the one or more clock signals may be provided to devices or communicatedbetween devices. For example, an external clock signal may be providedor a clock signal may be communicated over a link that couples atransmitting device and a receiving device.

The devices in these communication systems may include frequencysynthesizer circuitry to generate or modify the one or more clocksignals. For example, the frequency synthesizer circuitry may select oradjust a phase or a frequency of the one or more clock signals that areprovided to the devices. The output clock signals from this frequencysynthesizer are then coupled to additional circuitry, such as transmitor receive circuits, in the devices. This approach is referred to as aclock-forwarding architecture.

Unfortunately, many systems and devices that include a clock-forwardingarchitecture are sensitive to timing drift effects, such as thoseassociated with path-length differences, as well as process, voltageand/or temperature variations. For example, timing drifts may occur at avariety of locations in a respective device, including in the frequencysynthesizer (such as in a phase-frequency detector or a charge pump),over signal lines and wires that couple the additional circuitry to thefrequency synthesizer (such as clock-path mismatch, which gives rise toclock skew), and/or in the additional circuitry (such as in sample andhold circuits in a receiver). Timing drift may cause timing offsetbetween clock and data. If this timing offset is not corrected, systemor device performance (such as a bit-error rate) may be degraded.

In some existing systems and devices, open-loop circuit matching of apath length and/or one or more components are used to reduce oreliminate timing drift. However, this approach is limited by an accuracyof the matching. In more advanced processes, this matching may bedifficult to achieve, or it may be realized at a prohibitive power andarea penalty.

Closed-loop or continuous time timing drift cancellation techniquesoften offer better performance than open-loop matching. These approachesmay be able to eliminate or significantly reduce timing drift. However,existing closed-loop techniques often incur significant increases inpower consumption and overhead (such as significant circuit redundancyin the frequency synthesizer for each transmit or receiver circuit). Theexisting closed-loop techniques may also need a minimum transitiondensity in the data that is communicated. If this is achieved usingcoding there may be a reduction in an efficiency of the link. And if aperiodic timing calibration technique is used, there will be datainterrupts that may not be acceptable in certain applications.

There is a need, therefore, for improved timing drift cancellationcircuits and techniques that reduce and/or eliminating timing driftwithout the aforementioned problems.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, reference should be made to the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 schematically illustrates an eye pattern.

FIG. 2 is a block diagram illustrating an embodiment of a device.

FIG. 3 is a block diagram illustrating an embodiment of a device.

FIG. 4 is a block diagram illustrating an embodiment of a phase mixer.

FIG. 5 is a block diagram illustrating an embodiment of a receiver.

FIG. 6A is a block diagram illustrating an embodiment of a receiver.

FIG. 6B is a block diagram illustrating an embodiment of a receiver.

FIG. 7 is a flow diagram illustrating an embodiment of a method oftiming drift cancellation.

FIG. 8 is a flow diagram illustrating an embodiment of a method oftiming drift cancellation.

FIG. 9 is a block diagram illustrating an embodiment of a system.

Like reference numerals refer to corresponding parts throughout thedrawings.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of a circuit are described. In one embodiment, the circuitincludes N phase mixers to receive a primary clock signal, where arespective phase mixer in the N phase mixers is to output a respectivesecondary clock signal having a corresponding phase. The circuit alsoincludes a plurality of M receivers, where M is smaller than N. The Mreceivers are to receive data in accordance with respective secondaryclock signals from respective ones of a first group of M phase mixers inthe N phase mixers in a first time period, and are to receive data inaccordance with respective secondary clock signals from respective onesof a second group of M phase mixers in the N phase mixers in a secondtime period. The second group of M phase mixers has at least one firstphase mixer that is not in the first group of M phase mixers, and thefirst group of M phase mixers has at least one second phase mixer thatis not in the second group of phase mixers. A calibration circuit in thecircuit is to calibrate a secondary clock signal output by the at leastone first phase mixer in the first time period and is to calibrate thesecondary clock signal output by the at least one second phase mixer inthe second time period. In some embodiments, M equals N-1.

In some embodiments, the circuit further includes coupling circuitry toselectively couple a respective phase mixer of the N phase mixers, atdistinct times, to a first respective receiver in the plurality of Mreceivers, a second respective receiver in the plurality of receivers,and the calibration circuit.

In some embodiments, the respective phase mixer of the N phase mixersincludes circuitry for generating a respective secondary clock inaccordance with a first phase when the respective phase mixer is coupledto the first respective receiver and for generating the respectivesecondary clock in accordance with a second phase when the respectivephase mixer is coupled to a second respective receiver. The calibrationcircuit is to adjust the first phase and the second phase when therespective phase mixer is coupled to the calibration circuit.

In some embodiments, the circuit further includes control logic, wherethe control logic is to select a coupling configuration for thesecondary clock signals output by the N phase mixers, the plurality of Mreceivers and the calibration circuit.

In some embodiments, the calibration circuit is to adjust the phase ofthe secondary clock of a selected phase mixer to account for clock driftduring operation of the circuit.

A respective receiver in the M receivers may be associated with twophase mixers in the N phase mixers. In some embodiments, a respectivereceiver in the M receivers includes first and second receiver unitscoupled to respective ones of first and second phase mixers in the Nphase mixers. The first and second receiver units are to sample dataaccording to respective secondary clock signals from respective ones ofthe first and second phase mixers. The calibration circuit is to adjustthe phase of the secondary clock from one of the first and second phasemixers in accordance with data output from the first and second receiverunits.

In some embodiments, the circuit further includes a frequencysynthesizer to receive a reference clock signal and to output theprimary clock signal based on the reference clock signal, and anadditional phase mixer coupled to the frequency synthesizer, where theadditional phase mixer is to provide a feedback reference signal.

In another embodiment, a method of calibrating clock signals for timingdrift is described. In the method, data is received in accordance withrespective clock signals from respective ones of a first group of Mphase mixers in N phase mixers during a first time period, and data isreceived in accordance with respective clock signals from respectiveones of a second group of M phase mixers in the N phase mixers during asecond time period. The second group of M phase mixers has at least onefirst phase mixer that is not in the first group of M phase mixers, andthe first group of M phase mixers has at least one second phase mixerthat is not in the second group of M phase mixers. The method furtherincludes calibrating the at least one first phase mixer during the firsttime period, and calibrating the at least one second phase mixer duringthe second time period.

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings. In the following detaileddescription, numerous specific details are set forth in order to providea thorough understanding of the subject matter presented herein.However, it will be apparent to one of ordinary skill in the art thatthe subject matter may be practiced without these specific details. Inother instances, well-known methods, procedures, components, andcircuits have not been described in detail so as not to unnecessarilyobscure aspects of the embodiments.

A circuit that implements closed-loop or continuous clock data recoveryis described. The circuit may be included in an integrated circuit, andmay reduce and/or eliminate timing drift on a pin-by-pin or bit-by-bitbasis (in devices that include a multiple bit interface). The continuousclock data recovery utilizes limited or minimum circuit overhead, andmay avoid modifications to data coding (i.e., reductions in linkefficiency) and/or interrupts in data communication. This continuousclock data recovery approach utilizes one or more additional phasemixers in a set of phase mixers that are coupled to correspondingreceivers in a set of receivers.

In an exemplary embodiment, there are N phase mixers and N-1 receivers.A respective receiver is coupled to two of the N phase mixers, and at agiven time the respective receiver receives data in accordance with aclock signal from one of these two phase mixers. One phase mixer in theset of phase mixers is coupled to a calibration circuit by controllogic, where one or more timing offset values for at least one clocksignal output by the one phase mixer are determined. The one or moretiming offset values may correspond to a phase of the clock signal, andmay be used to set a sampling time corresponding to the clock signal atapproximately the center of an eye pattern thereby reducing and/oreliminating timing drift. During a time interval, all of the clocksignals that are output from the phase mixers to the receivers may becalibrated by coupling the clock signals to the calibration circuit in around-robin fashion. Thus, each of the clock signals is coupled to thecalibration circuit during different subintervals in the time interval.

In some embodiments, the calibration of the clock signals uses pairs ofreceivers, where one of the receivers receives data in accordance with afirst calibrated clock signal and the other receiver receives data inaccordance with a second clock signal that is to be calibrated. In thisapproach, the calibration of the second clock signal may be inaccordance with the data received by the two receivers.

The circuit may be a memory controller and/or a memory device. Thememory device may include a memory core that utilizes solid-statememory, semiconductor memory, organic memory and/or another memorymaterial, including volatile and/or non-volatile memory. The memorydevice may include dynamic random access memory (DRAM), static randomaccess memory (SRAM) and/or electrically erasable programmable read-onlymemory (EEPROM). The circuit may be included in one or more componentsin a memory system, such as a memory controller and/or one or morememory devices. The one or more memory devices may be embedded in one ormore memory modules. The memory controller and the one or more memorydevices may be on a common or same circuit board. The circuit may beincluded in one or more components in other systems, such as those thatinclude logic chips, including a serializer/deserializer, PCI Expressand/or other high-speed interfaces or input/output links.

FIG. 1 provides a schematic of an eye diagram or eye pattern 100associated with a data signal. The eye diagram 100 indicates anacceptable range of timing values that is defined by pass(P) and fail(F)boundaries 110 and 112. A timing drift t_(drift) 114 is shown withrespect to the center of the eye diagram 100. In a 5 Gb/s link that hasa 200 ps bit time, t_(drift) 114 of 20 ps will reduce the timing marginby 40 ps (twice t_(drift) 114). In one embodiment of the circuit, theclock signal used by a respective receiver to receive a data signal iscalibrated to be near a center of the eye diagram associated with thedata signal.

Attention is now directed towards embodiments that address thedifficulties associated with existing approaches for clock datarecovery. FIG. 2 is a block diagram illustrating an embodiment 200 of adevice 214. Device 214 includes receivers 222-1 through 222-N to receiverespective data signals 212-1 through 212-N, and a frequency synthesizer216 to receive a reference clock signal 210. Reference clock signal 210may be forwarded from another device or locally generated. The frequencysynthesizer 216, which may include a phase locked loop (PLL) and/or adelay locked loop (DLL), outputs one or more primary clock signals thatcorrespond to the reference clock signal 210. The one or more primaryclock signals may have different phases and/or frequencies than thereference clock signal 210. In an exemplary embodiment, the primaryclock signals include a set of phasor signals that are offset in phasefrom each other by an integer multiple of a predetermined phasedifference. For example, the primary clock signals may include eightphasor signals and the predetermined phase difference may beapproximately 45 degrees (i.e., approximately one eighth of 360degrees).

The primary clock signals are coupled to 2N phase mixers 220. The phasemixers 220 each generate a secondary clock signal in accordance with oneor more of the primary clock signals, for example, by interpolatingbetween two respective phasors. The secondary clock signals may becalibrated during an initialization or start-up procedure, such as whenthe device 214 is first powered on. The calibration adjusts and/ordetermines a phase of a respective secondary clock signal such thatsampling times corresponding to the secondary clock signal are centeredwithin an eye pattern, such as the eye pattern 100 (FIG. 1). Forexample, one or more offset values for each secondary clock signal maybe determined. These initial offset values may be used by correspondingphase mixers 220 when generating the secondary clock signals. Inaddition, an optional phase mixer 218 may provide a feedback referenceclock signal to the frequency synthesizer 216 to assist the frequencysynthesizer 216 in maintaining a frequency and/or a phase lock. Thisfeedback mechanism may not be necessary, however, since the phase mixers220 may be calibrated for the timing drifts in the device 214.

The 2N phase mixers may be coupled to a set of N receivers 222. In anexemplary embodiment, two of the phase mixers 220 are coupled to acorresponding one of the receivers 222. The receivers 222 receive datasignals 212 in accordance with the secondary clock signals output bycorresponding phase mixers 220. For example, at a given time a samplingtime of a respective receiver (such as receiver 222-1) may correspond toa secondary clock signal from a first of the two phase mixers 220 thatare coupled to the respective receiver. Simultaneously, a secondaryclock signal from a second of the two phase mixers 220 that are coupledto the respective receiver may be calibrated for timing drift. At alater time, after the calibration of the first phase mixer is completed,the sampling time of the respective receiver may correspond to thesecondary clock signal from the second mixer and the secondary clocksignal from the first mixer may be calibrated for timing drift. 100371In this way, the clock data recovery approach illustrated in theembodiment 200 may reduce and/or eliminate timing drift that occursafter the initial calibration of the device 214 (such as timing driftsthat occur during operation) thereby maintaining the communicationchannel error margin. However, this approach may entail considerableoverhead (such as a doubling of the number of phase mixers 220) with acommensurate increase in circuit area and power consumption.

FIG. 3 is a block diagram illustrating an embodiment 300 of a device 310that includes the ability to recalibrate the secondary clock signalswhile significantly reducing the associated overhead. Device 310includes a set of N phase mixers 318 and a set of N-1 receivers 314. Inan exemplary embodiment, there are 9 phase mixers 318 and 8 receivers314 (corresponding to a data byte that is communicated over aninterface). In the exemplary embodiment, the reference clock signal 210may have a frequency of 500 MHz and the primary clock signals (the setof phasors) output by the frequency synthesizer 216 may have a frequencyof 2.5 GHz.

In one embodiment, at a given time a respective receiver 314 is toreceive data in accordance with a secondary clock signal from one of twophase mixers 318. The secondary clock signal from another one of the twophase mixers 318 may be coupled to a calibration circuit 320 or toanother receiver 314. The calibration circuit 320 may determine one ormore clock signal timing offset values and/or one or more receivercircuit voltage offset values. The timing offset values may correspondto a phase of the one of the secondary clock signals and may centersampling times corresponding to the secondary clock signal at the centerof an eye pattern. The phase mixers 318 are discussed further below withreference to FIG. 4.

The secondary clock signals coupled to the corresponding receivers 314may be changed, and therefore each of the secondary clock signals may becalibrated from time to time. For example, the secondary clock signalsmay be coupled to the calibration circuit 320 in a round-robin fashionso that the secondary clock signals are calibrated one by one insuccessive time intervals. Thus, calibration of the secondary clocksignals may be performed on the fly, i.e., without interrupting datacommunication. In an exemplary embodiment, each time interval may bebetween 1 μs and 1 ms.

In some embodiments, the configuration of phase mixers 318 providingsecondary clock signals to the receivers 314 may be in accordance withcontrol signals from control logic 316. For example, the secondary clocksignals may be selectively coupled to the receivers 314 using one ormore multiplexers. Embodiments of circuits and related methods forcalibrating the secondary clock signals are discussed further below withreference to FIGS. 5-8.

Note that while the embodiment 200 (FIG. 2) and the embodiment 300 focuson reduction and/or elimination of timing drift in receivers 222 (FIG.2) and 314, timing drift in transmitters (elsewhere in a system) may be,at least in part, addressed by including a transmit clock in a feedbackloop, such as a feedback loop that includes a PLL. This transmit clockmay be shared by more than one transmitter. Residual timing drift orclock skew in the transmitter may be associated with a phase-frequencydetector and/or a charge pump in the PLL. This residual timing drift maybe reduced and/or eliminated in the receiver, as discussed further belowwith reference to FIGS. 6 and 8.

In some embodiments, the device 310 may include fewer or additionalcomponents. For example, in some embodiments the device 310 may includean optional divide-by-N component 312, such as a divide-by-5 component,in the feedback loop for the frequency synthesizer 216. Logicalpositions of one or more components in the device 310 may be changed,and two or more of the components may be combined and/or shared.

FIG. 4 is a block diagram illustrating an embodiment 400 of a phasemixer 408, such as one of the phase mixers 318 (FIG. 3). The phase mixer408 may receive primary clock signals, such as a set of phasors 410,from the frequency synthesizer 216 (FIG. 3). A selective coupler, suchas multiplexer 418, may couple a respective pair of phasors to a mixer420 in accordance with a phase value or command from control logic 412.The mixer 420 may generate a secondary clock signal by interpolatingbetween the respective pair of phasors in accordance with the phasevalue or command from the control logic 412. In some embodiments,therefore, the secondary clock signal may correspond to one or more ofthe phasors.

The secondary clock signal, as well as secondary clock signals 424 fromother phase mixers, may be coupled to corresponding receivers 426 usinga router 422. As described further below with reference to FIGS. 5 and6, in some embodiments, however, multiplexers may be used in addition toor in place of the router 422. In some embodiments, the router 422 orthe multiplexers are not used and a respective receiver is coupled totwo of the phase mixers. In these embodiments, the respective receiverincludes selection circuitry controlled by control logic 316 to select asecondary clock signal from one of the two phase mixers, or to select adata sample obtained according to one of the secondary clock signalsfrom the two phase mixers, as discussed below.

Selective coupling by the multiplexer 418 may be in accordance withcontrol signals from control logic 316 (FIG. 3) and interpolation in themixer 420 may be in accordance with one or more offset values stored inregisters 414 and 416. For example, at a given time the phase mixer 408may provide one of the secondary clock signals to a respective receiver,such as a first receiver that corresponds to a bit k in a byte. A phaseof the secondary clock signal may be determined and/or selected inaccordance with registers 414. In particular, register 414-1 may includean offset value (offset 0) that is determined during initialization ofthe device 310 (FIG. 3), for example, when the device 310 is firstpowered on. Register 414-2 may include an offset value (offset cntr)that is updated during recalibration of the secondary clock signal forthe first receiver.

In one embodiment, the secondary clock signal from the phase mixer 408is provided to two of the receivers 314, as shown in FIG. 3. Forexample, the secondary clock signal may also be provided to a secondreceiver that corresponds to bit k+1 in the byte. In some embodiments,the phase mixer 408 may include two mixers 420 to output two secondaryclock signals, including one for the receiver corresponding to bit k andanother fore the receiver corresponding to bit k+1. Alternatively, atcertain times during the time interval, one of the two secondary clocksignals may be coupled to the calibration logic 320 (FIG. 3) forrecalibration. The calibration logic 320 (FIG. 3) may determine anupdate to an offset value, such as the offset value stored inoffset-cntr register 416-2.

In some embodiments, the phase mixer 408 may include fewer or additionalcomponents. Logical positions of one or more components in the phasemixer 408 may be changed, and two or more of the components may becombined and/or shared. Also, two or more phase mixers may be combinedinto one phase mixer to output two or more independent secondary clocksignals.

FIG. 5 is a block diagram illustrating an embodiment 500 of a receiver,such as one of the receivers 314 (FIG. 3). Secondary clock signals 510from two phase mixers 314 (FIG. 3) may be selectively coupled usingmultiplexers 512 to receiver circuits 514 and the calibration circuit320. Receiver circuits 514 may receive data at sample times thatcorrespond to one or both edges in a respective secondary clock signalthat is coupled to the receiver circuits 514. Thus, in some embodiments,the receiver circuits 514 have a dual data rate mode of operation.

The calibration circuit 320 may include a phase detector 516 and a phasecontrol unit 520. The phase detector 516 may determine a phasedifference between a respective secondary clock signal that is coupledto the phase detector 516 and a reference clock signal, such as aphasor/N signal 518. The phasor/N signal 518 may correspond to an outputfrom the optional divide-by-N component 312 (FIG. 3). An output from thephase detector 516 may be coupled to the phase control unit 520, whichmay output one or more updates 522 to update offset values for one ormore offset registers (such as registers 414-2 and 416-2 in FIG. 4).

At a later time, the respective secondary clock signals coupled to thereceiver circuits 514 and the calibration logic 320 may be changed,thereby allowing another secondary clock signal to be recalibrated. Insome embodiments, embodiment 500 may include fewer or additionalcomponents, logical positions of one or more components may be changed,and two or more of the components may be combined and/or shared.

In the receiver embodiment 600 shown in FIG. 6A, the receiver includesfirst and second receiver units 601 and 602, one of which, such asreceiver unit 601 is to output received data “Rdata”. Each receiver unit601 or 602 includes at least one receiver circuit 612. For example,receiver unit 601 includes receiver circuits 612-1 and 612-2, andreceiver unit 602 includes receiver circuits 612-3 and 612-4. Secondaryclock signals 610 from two phase mixers 314 (FIG. 3) may be selectivelycoupled using multiplexers 608 to receiver unit 601 and receiver unit602. Each receiver unit 601 or 602 may receive data at sample times thatcorrespond to one or both edges in a respective secondary clock signal610 that is coupled to that receiver unit. In addition, in someembodiments receiver units 601 and 602 are multi-bit or multi-symbolreceivers (i.e., receivers that receive signals representing more thanone bit per sample time), while in other embodiments the receiver units601 and 602 are single-bit or single-symbol receivers (receivers thatreceive signals representing a single bit or symbol per sample time). Asin embodiment 500, a respective secondary clock signal is coupled to tworeceivers of two different bit streams. For example, secondary clocksignal 610-1 is coupled to the receiver for bit or symbol J−1 and to thereceiver 622 for bit or symbol J. Similarly, secondary clock signal610-2 is coupled to the receiver 622 for bit J and to the receiver forbit or symbol J+1. The receiver for bit or symbol J−1 or J+1 may besimilarly configured as receiver 622 for bit or symbol J and thus alsoincludes first and second receiver units. Therefore, in embodiment 600,there may be twice as many receivers 612 as needed for each data bit orsymbol stream.

Data output by the receiver circuits 612 may be converted from serial toparallel format in serial-to-parallel converters 614. For example,serial data input to the serial-to-parallel converters 614 maycorrespond to 2.5 GHz data streams and the parallel data output maycorrespond to ten parallel 250 MHz data streams. The data may be coupledto calibration logic 618 (discussed further below).

While FIG. 6A shows receiver circuitry 622-1 for only one bit or symbolstream (e.g., 1-bit, 2-bit, 3-bit or 4-bit symbols transmitted using2-PAM, 4-PAM, 8-PAM or 16-PAM signals), a complete system may haveadditional instances of the receiver circuitry 622 for each additionalbit or symbol that is received in parallel by the system or circuit.Each of those receiver circuits 620 outputs data to the calibrationlogic 618.

The respective secondary clock signal, such as the secondary clocksignal 610-1, that is coupled to the receivers 612-1 and 612-2 mayalready be calibrated. For example, this secondary clock signal may havebeen previously calibrated using the approach illustrated in embodiment500 (FIG. 5) and/or embodiment 600. Thus, first data received by thereceiver circuits 612-1 and 612-2 may be reliable or trusted data, i.e.,data that is deemed correct.

This trusted first data may be used to calibrate another respectivesecondary clock signal, such as the secondary clock signal 610-2, thatis coupled to the receiver circuits 612-3 and 612-4. For example, seconddata received by the receiver circuits 612-1 and 612-4 may be comparedto the trusted first data in the calibration logic 618. In an exemplaryembodiment, the comparison includes an XOR operation. A phase of thesecondary clock signal 610-2 that is being calibrated may besystematically swept over the corresponding eye pattern such as the eyepattern 100 (FIG. 1), in order to determine pass-fail boundaries, suchas the pass-fail boundaries 110 and 112 (FIG. 1). In this way, samplingtimes corresponding to the secondary clock signal 610-2 may be centeredin the eye pattern and the channel error margin may be increased and/ormaximized. Techniques such as flex phase and schmooing may be utilizedin this process. Upon completion of the calibration or recalibration,updates to offset register 414-2 and/or 416-2 (FIG. 4) may be providedto one or more of the phase mixers 318 (FIG. 3). In addition, a voltageoffset of a respective receiver circuit 612 (e.g., 612-1) in a receiverunit 601 or 602 (of a receiver 622) may be systematically swept over thecorresponding eye pattern in order to determine pass-fail boundaries,and to then select a center or best voltage offset for the respectivereceiver circuit 612. In some embodiments, the resulting selectedvoltage offset is stored as a digital value in a register of thereceiver unit 601 or 602, or elsewhere in the receiver 600, while inother embodiments the resulting selected voltage offset is stored as ananalog signal. The voltage offset calibration process can be performedfor all the receiver circuits 612 of a respective receiver 622. It isnoted that receiver 600 recalibrates the voltage offsets of a respectivereceiver circuit 612 in a respective receiver unit 601 or 602 bycomparing the data produced by the respective receiver circuit 612 withthe reliable or trusted data produced by the other receiver unit 602 or601 in the same receiver 622.

At a later time, the respective secondary clock signals coupled to thereceiver units 601 and 602 may be changed, thereby allowing thesecondary clock signal 610-1 to be recalibrated. During recalibration ofsecondary clock signal 610-1, the other secondary clock signal 610-2(which has already been recalibrated) is used to obtain reliable ortrusted data that are used to calibrate the secondary clock signal610-1.

In some embodiments, the secondary clock signal 610-2 may be coupled toan additional receiver circuit 622 along with yet another secondaryclock signal 610-x. Each of these secondary clock signals is used duringa respective time period to obtain reliable or trusted data that areused to calibrate the other secondary clock signal.

FIG. 6B is a block diagram illustrating an embodiment 650 of a receiver662, such as one of the receivers 314 (FIG. 3) Embodiment 650 is similarto embodiment 600 except that multiplexers 608 are removed andmultiplexer 616 is added to select one of serial-to-parallel converters614 to output Rdata. As a result, only one multiplexer 616 is required,and secondary clock signals 610-1 and 610-2 can be directly coupled torespective ones of the receiver units 601 and 602. As in the embodiment600, there are twice as many receiver units 601 and 602 as needed foreach data bit or symbol stream, and a respective secondary clock signalis coupled to two receivers, which receive of two different bit streams.For example, secondary clock signal 610-1 is coupled to two receivers622, one for receiving bit or symbol J, and another for receiving bit orsymbol J−1. Similarly, secondary clock signal 610-2 is coupled to tworeceivers 622, one for receiving bit or symbol J and another forreceiving bit or symbol J+1.

While the approach illustrated in FIGS. 6A and 6B may entail additionaloverhead, these approaches offer the advantage of correcting for timingdrift and/or voltage offsets in both a transmitter and a receiver, i.e.,in the entire communication channel. In some embodiments, a minimumtransition density in the first and second data may be necessary inorder to use the approach illustrated in embodiments 600 and/or 650.This may be achieved by scrambling and/or encoding the first and seconddata. In other embodiments, however, the approaches illustrated in FIG.5, and the approaches illustrated in FIGS. 6A and 6B may both beutilized. For example, the approach illustrated in FIG. 5 may be usedduring initialization of the circuit and/or when the transition densityis less than a threshold value, for example a transition density of lessthan one data edge per thousand bit times or symbol times. The thresholdtransition density value may vary with system voltage and temperature.The approach illustrated in FIG. 6A or 6B may be used duringrecalibration of the secondary clock signals and/or when the transitiondensity is greater than the threshold value. In other embodiments, thetwo approaches may be used simultaneously.

In some embodiments, the embodiments shown in FIGS. 6A and/or 6B 600 mayinclude fewer or additional components, logical positions of one or morecomponents may be changed, and two or more of the components may becombined and/or shared.

There are a variety of coupling configurations that may be used duringinitialization and calibration of the secondary clock signals. Tables I,II and III-XI illustrate examples of embodiments of couplingconfigurations during initialization and normal operation for the device310 shown in FIG. 3. Tables I and II show a first and second couplingconfiguration that may be used during initialization. Tables III-XI showa sequence of coupling configurations that may be used duringcorresponding time intervals after the initialization.

TABLE I An embodiment of a first coupling configuration duringinitialization of a circuit. Phase Mixer Configuration 0 Receiver 0 1Receiver 1 2 Receiver 2 3 Receiver 3 4 Receiver 4 5 Receiver 5 6Receiver 6 7 Receiver 7

TABLE II An embodiment of a second coupling configuration that is usedafter the first coupling configuration during initialization of acircuit. Phase Mixer Configuration 1 Receiver 0 2 Receiver 1 3 Receiver2 4 Receiver 3 5 Receiver 4 6 Receiver 5 7 Receiver 6 8 Receiver 7

TABLE III An embodiment of a coupling configuration in a first timeinterval. Phase Mixer Configuration 1 Receiver 0 2 Receiver 1 3 Receiver2 4 Receiver 3 5 Receiver 4 6 Receiver 5 7 Receiver 6 8 Receiver 7 0Calibration Logic

TABLE IV An embodiment of a coupling configuration in a second timeinterval. Phase Mixer Configuration 0 Receiver 0 2 Receiver 1 3 Receiver2 4 Receiver 3 5 Receiver 4 6 Receiver 5 7 Receiver 6 8 Receiver 7 1Calibration Logic

TABLE V An embodiment of a coupling configuration in a third timeinterval. Phase Mixer Configuration 0 Receiver 0 1 Receiver 1 3 Receiver2 4 Receiver 3 5 Receiver 4 6 Receiver 5 7 Receiver 6 8 Receiver 7 2Calibration Logic

TABLE VI An embodiment of a coupling configuration in a fourth timeinterval. Phase Mixer Configuration 0 Receiver 0 1 Receiver 1 2 Receiver2 4 Receiver 3 5 Receiver 4 6 Receiver 5 7 Receiver 6 8 Receiver 7 3Calibration Logic

TABLE VII An embodiment of a coupling configuration in a fifth timeinterval. Phase Mixer Configuration 0 Receiver 0 1 Receiver 1 2 Receiver2 3 Receiver 3 5 Receiver 4 6 Receiver 5 7 Receiver 6 8 Receiver 7 4Calibration Logic

TABLE VIII An embodiment of a coupling configuration in a sixth timeinterval. Phase Mixer Configuration 0 Receiver 0 1 Receiver 1 2 Receiver2 3 Receiver 3 4 Receiver 4 6 Receiver 5 7 Receiver 6 8 Receiver 7 5Calibration Logic

TABLE IX An embodiment of a coupling configuration in a seventh timeinterval. Phase Mixer Configuration 0 Receiver 0 1 Receiver 1 2 Receiver2 3 Receiver 3 4 Receiver 4 5 Receiver 5 7 Receiver 6 8 Receiver 7 6Calibration Logic

TABLE X An embodiment of a coupling configuration in a eighth timeinterval. Phase Mixer Configuration 0 Receiver 0 1 Receiver 1 2 Receiver2 3 Receiver 3 4 Receiver 4 5 Receiver 5 6 Receiver 6 8 Receiver 7 7Calibration Logic

TABLE XI An embodiment of a coupling configuration in a ninth timeinterval. Phase Mixer Configuration 0 Receiver 0 1 Receiver 1 2 Receiver2 3 Receiver 3 4 Receiver 4 5 Receiver 5 6 Receiver 6 7 Receiver 7 8Calibration Logic

Attention is now directed towards processes for reducing and/oreliminating timing drift. FIG. 7 is a flow diagram illustrating anembodiment of a method 700 of timing drift cancellation. This method 700may correspond to embodiment 500 (FIG. 5). First phase offset values forclock signals in a plurality of clock signals are initialized using areference clock (710). Second phase offset values for the clock signalsin the plurality of clock signals are initialized using received datasignals. In some embodiments, the received data signals are used to formor simulate an eye diagram and the phase offset values are initializedwith respect to the center of the eye pattern (712). Operations 710 and712 are used to set initial values for the timing or phase offsetsstored in the phase mixer offset registers (see FIG. 4). A group of Mclock signals in the plurality of clock signals are coupled torespective ones of M receivers(714). Another clock signal in theplurality of clock signals (i.e., a clock signal that is not in thegroup of M clock signals) is calibrated for timing drift by determiningan update to a respective first offset value using the reference clock(716). The selective coupling and calibration operations are repeatedduring a time interval until all of the clock signals in the pluralityof clock signals are calibrated for timing drift (718). In someembodiments, there may be fewer or additional operations, an order ofthe operations may be rearranged and/or two or more operations may becombined. In embodiments in which a respective clock signal isselectively coupled to a plurality of distinct respective receivers (oneat a time) at different times, distinct offsets are determined for therespective clock signal for each receiver to which it may be coupled.

FIG. 8 is a flow diagram illustrating an embodiment of a method 800 oftiming drift cancellation. This method 800 may correspond to thereceiver embodiments shown in FIGS. 6A and/or FIG. 6B. A first clocksignal in a plurality of clock signals us calibrated for timing drift(810). A first clock signal is selectively coupled to a first receiverunit in a first receiver and a second clock signal is selectivelycoupled to a second receiver unit in the first receiver (812). A timingdrift in the second clock signal may be calibrated using data receivedfrom the first and second receiver units in the first receiver (814).The second clock signal is selectively coupled to a first receiver unitin a second receiver and a third clock signal is selectively coupled toa second receiver unit in the second receiver (816). For example, thefirst receiver may be the receiver 622 in FIG. 6A or the receiver 662 inFIG. 6B, and the second receiver may be the receiver for bit or symbolj+1 in FIG. 6A or 6B. A timing drift in the third clock signal may becalibrated using data received from the first and second receiver unitsin the second receiver(818). In some embodiments, there may be fewer oradditional operations, an order of the operations may be rearrangedand/or two or more operations may be combined.

Devices and circuits described herein can be implemented using computeraided design tools available in the art, and embodied by computerreadable files containing software descriptions of such circuits, atbehavioral, register transfer, logic component, transistor and layoutgeometry level descriptions stored on storage media or communicated bycarrier waves. Data formats in which such descriptions can beimplemented include, but are not limited to, formats supportingbehavioral languages like C, formats supporting register transfer levelRTL languages like Verilog and VHDL, and formats supporting geometrydescription languages like GDSII, GDSIII, GDSIV, CIF, MEBES and othersuitable formats and languages. Data transfers of such files on machinereadable media including carrier waves can be done electronically overthe diverse media on the Internet or through email, for example.Physical files can be implemented on machine readable media such as 4 mmmagnetic tape, 8 mm magnetic tape, 3½ inch floppy media, CDs, DVDs andso on.

FIG. 9 is a block diagram illustrating an embodiment of a system 900 forstoring computer readable files containing software descriptions of thecircuits. The system 900 may include at least one data processor orcentral processing unit (CPU) 910, memory 914 and one or more signallines or communication busses 912 for coupling these components to oneanother. Memory 914 may include high-speed random access memory and/ornon-volatile memory, such as one or more magnetic disk storage devices.Memory 914 may store a circuit compiler 916 and circuit descriptions918. Circuit descriptions 918 may include circuit descriptions for thecircuits, or a subset of the circuits discussed above with respect toFIGS. 3-5. In particular, circuit descriptions 918 may include circuitdescriptions of one or more receivers 920, one or more control logiccircuits 922, one or more frequency synthesizers 924, one or moredivide-by-N circuits 926, one or more phase mixers 928, one or moremultiplexers 930, one or more phase detectors 932, one or more phasecontrol units 934, calibration logic 936, and one or moreserial-to-parallel converters 938.

The foregoing descriptions of specific embodiments of the presentinvention are presented for purposes of illustration and description.They are not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Rather, it should be appreciated that manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated.

1. A circuit, comprising: N phase mixers to receive a primary clocksignal, wherein a respective phase mixer in the N phase mixers is tooutput a respective secondary clock signal having a corresponding phase;and a plurality of M receivers, wherein M is smaller than N, and whereinthe M receivers are to receive data in accordance with respectivesecondary clock signals from respective ones of a first group of M phasemixers in the N phase mixers in a first time period, and to receive datain accordance with respective secondary clock signals from respectiveones of a second group of M phase mixers in the N phase mixers in asecond time period, the second group of M phase mixers having at leastone first phase mixer that is not in the first group of M phase mixers,the first group of M phase mixers having at least one second phase mixerthat is not in the second group of phase mixers; and a calibrationcircuit to calibrate a secondary clock signal output by the at least onefirst phase mixer in the first time period and to calibrate thesecondary clock signal output by the at least one second phase mixer inthe second time period.
 2. The circuit of claim 1, wherein M equals N-1.
 3. The circuit of claim 1, further comprising coupling circuitry toselectively couple a respective phase mixer of the N phase mixers, atdistinct times, to a first respective receiver in the plurality of Mreceivers, a second respective receiver in the plurality of receivers,and the calibration circuit.
 4. The circuit of claim 3, wherein therespective phase mixer of the N phase mixers includes circuitry forgenerating a respective secondary clock in accordance with a first phasewhen the respective phase mixer is coupled to the first respectivereceiver and for generating the respective secondary clock in accordancewith a second phase when the respective phase mixer is coupled to asecond respective receiver.
 5. The circuit of claim 4, wherein thecalibration circuit is to adjust the first phase and the second phasewhen the respective phase mixer is coupled to the calibration circuit.6. The circuit of claim 1, further comprising control logic, wherein thecontrol logic is to select a coupling configuration for the secondaryclock signals output by the N phase mixers, the plurality of M receiversand the calibration circuit.
 7. The circuit of claim 1, wherein thecalibration circuit is to adjust the phase of the secondary clock of aselected phase mixer to account for clock drift during operation of thecircuit.
 8. The circuit of claim 1, wherein a respective receiver in theM receivers is associated with two phase mixers in the N phase mixers.9. The circuit of claim 1, wherein a respective receiver in the Mreceivers includes first and second receiver units coupled to respectiveones of first and second phase mixers in the N phase mixers, and whereinthe first and second receiver units are to sample data according torespective secondary clock signals from respective ones of the first andsecond phase mixers.
 10. The circuit of claim 9, wherein the calibrationcircuit is to adjust the phase of the secondary clock from one of thefirst and second phase mixers in accordance with data output from thefirst and second receiver units.
 11. The circuit of claim 1, furthercomprising a frequency synthesizer to receive a reference clock signaland to output the primary clock signal based on the reference clocksignal, and an additional phase mixer coupled to the frequencysynthesizer, wherein the additional phase mixer is to provide a feedbackreference signal.
 12. A circuit, comprising: first means for receiving areference clock signal outputting a primary clock signal; a plurality ofsecond means coupled to the frequency synthesizer, wherein a respectivesecond means in the plurality of second means is for outputting arespective secondary clock signal having a corresponding phase; and aplurality of third means, wherein a respective third means in theplurality of third means is coupled to two of the second means, andwherein at a respective time the respective third means is for receivingdata in accordance with a respective secondary clock signal from one ofthe two second means coupled to the respective third means; and fourthmeans for calibrating a secondary clock signal output by a respectivesecond means by adjusting the phase of the secondary clock signal of therespective second means.
 13. A computer readable medium containing datarepresenting a circuit that includes: a frequency synthesizer, whereinthe frequency synthesizer is to receive a reference clock signal and isto output a primary clock signal; N phase mixers coupled to thefrequency synthesizer, wherein a respective phase mixer in the N phasemixers is to output a respective secondary clock signal having acorresponding phase; and a plurality of receivers, wherein a respectivereceiver in the plurality of receivers is coupled to two of the N phasemixers, and wherein at a respective time the respective receiver is toreceive data in accordance with the respective secondary clock signalfrom one of the two phase mixers coupled to the respective receiver; anda calibration circuit to calibrate a secondary clock signal output by arespective phase mixer in the N phase mixers by adjusting the phase ofthe secondary clock signal of the respective phase mixer.
 14. A methodof calibrating clock signals for timing drift, comprising: receivingdata in accordance with respective clock signals from respective ones ofa first group of M phase mixers in N phase mixers during a first timeperiod; receiving data in accordance with respective clock signals fromrespective ones of a second group of M phase mixers in the N phasemixers during a second time period, wherein the second group of M phasemixers has at least one first phase mixer that is not in the first groupof M phase mixers, and wherein the first group of M phase mixers has atleast one second phase mixer that is not in the second group of M phasemixers; calibrating the at least one first phase mixer during the firsttime period; and calibrating the at least one second phase mixer duringthe second time period
 15. The method of claim 14, wherein a pluralityof receivers that receive the data include less than N receivers thatare each coupled to two of the N phase mixers.
 16. The method of claim14, wherein N- I receivers receive the data.
 17. The method of claim 14,further comprising adjusting a phase of a respective clock signal suchthat a sampling time corresponding to the respective clock signal isapproximately at a center of an eye pattern.
 18. The method of claim 14,further comprising selectively coupling a respective phase mixer of theN phase mixers, at distinct times, to a first respective receiver in aplurality of receivers, a second respective receiver in the plurality ofreceivers, and a calibration circuit.
 19. The method of claim 18,further comprising generating a respective clock signal at a respectivephase mixer of the N phase mixers in accordance with a first phase ofthe respective phase mixer when the respective phase mixer is coupled toa first receiver in a plurality of receivers and for generating therespective secondary clock in accordance with a second phase of therespective phase mixer when the respective phase mixer is coupled to asecond receiver in the plurality of receivers.
 20. The method of claim19, further comprising determining the first phase in accordance with afirst offset between the respective clock signal and a reference clocksignal, and determining the second phase in accordance with a secondoffset between the respective clock signal and the reference clocksignal.
 21. The method of claim 19, further comprising: determining thefirst phase such that sampling times at the first receiver, whilecoupled to the respective clock signal, are approximately at a center ofan eye pattern associated with the data received by the first receiver;and determining the second phase such that sampling times at the secondreceiver, while coupled to the respective clock signal, areapproximately at a center of an eye pattern associated with the datareceived by the second receiver.
 22. The method of claim 19, furthercomprising determining an offset of the respective clock signal inaccordance with first data received at the first receiver using therespective clock signal and second data received using another clocksignal that has been calibrated for timing drift, wherein the phase ofthe respective clock signal is further adjusted in accordance with theoffset.
 23. The method of claim 22, further comprising determining avoltage offset of the first receiver in accordance with the first datareceived using the respective clock signal and the second data receivedusing the other clock signal that has been calibrated for timing drift,wherein a voltage of the first receiver is adjusted in accordance withthe voltage offset.